A conditional mutant retains the function of a gene under one set of conditions, called permissive, but lacks that function under a different set of conditions, called nonpermissive; the latter must still be permissive for the wild-type allele of the gene. Conditional mutants are presumed, in most cases, to result from missense mutations in a structural gene encoding a protein. In the case of temperature-sensitive (ts) mutants, the amino acid replacement resulting from the missense mutation partially destabilizes the encoded protein, resulting in the maintenance of its three-dimensional integrity only at relatively low temperatures.
Conditional mutants make possible the analysis of physiological changes caused by inactivation of a gene or gene product, and can be used to address the function of any gene. This strategy is especially valuable for the analysis of essential genes. Several types of conditional mutants and methods for producing them have been developed since the original demonstration of the utility of ts mutants (Horowitz, Genetics 33, 612 (1948); Horowitz, Adv. Genetics 3, 33(1950)) but the ts phenotype is still the one most frequently used.
One limitation of the ts approach is the uncertainty as to whether a given gene can be mutated to yield a ts product. For example, only six loci were identified after repeated searches for ts lethal mutations mapping to the S. cerevisiae chromosome I, which contains at least one hundred genes (more that six of which are essential) (Kaback et al., Genetics 108: 67 (1984); Harris and Pringle, Genetics 127: 279 (1991)). Another problem with conventional ts mutations is that they are often too "leaky" to be useful. That is, the function of a leaky ts protein at nonpermissive temperatures is not fully blocked by the mutation. For these and other reasons, a method for producing ts mutants which does not require a search for a ts mutation in a gene of interest would be extremely useful in a variety of applications.